Vehicles such as trucks, cars, scooters, tractors and most types of all-terrain vehicles (ATVs) are equipped with a mechanical continuously variable transmission (CVT) system. A continuously variable transmission (CVT) (also known as a single-speed transmission, stepless transmission, pulley transmission, or, in case of motorcycles, a twist-and-go) is an automatic transmission that can change seamlessly through a continuous range of effective gear ratios. This contrasts with other mechanical transmissions that offer a fixed number of gear ratios. The flexibility of a CVT allows the input shaft to maintain a constant angular velocity.
As presented in prior art FIG. 1, the CVT 100 system normally includes a primary pulley 102 mounted to the vehicle engine, a secondary pulley 104 connected to the vehicle propulsion assembly through some mechanical power transmission mean, and a drive belt 106 linking the pulleys and transferring the power from the primary pulley to the secondary pulley. The CVT 100 provides a continuous range of effective transmission ratio between the two pulleys in order to maintain an optimal engine rotational speed at a given rotational speed of the secondary pulley 104. As the vehicle accelerates or decelerates, the CVT 100 system varies the ratio accordingly. The ratio variation is achieved by changing the diameter at which the drive belt 106 winds around both pulleys. From a stopped state a vehicle CVT is required to transmit a great torque at a low speed. The winding diameter at the primary pulley 102 should then be much smaller than that of the secondary pulley 104, as presented by CVT 100 in minimum ratio position 108. Then, as the vehicle accelerates, the winding diameter of the primary pulley 102 increases while that of the secondary pulley 104 decreases, thereby transmitting a weaker torque to the propulsion assembly at a greater speed, as presented by CVT 100 in maximum ratio position 109. CVT system 100 can be driven by various power sources such as by a combustion engine, an electric motor, a turbine, etc. Moreover, CTV systems can also be used for driving other types of equipment such as conveyors, snow blowers, etc.
Each pulley (102 and 104) of the CVT 100 includes two main parts called sheaves (110 and 112) that are mounted, through a concentric hole, onto the pulley's supporting shaft 114. Each sheave has a conically shaped surface facing the opposite sheave forming a V-shaped circular groove between the pair of sheaves. The trapezoidal drive belt 106 that links both pulleys is seated midway in the grooves formed by the two opposite sheaves of each pulley. For each pulley, one 110 of the two sheaves is mechanically fixed to one end of the pulley's supporting shaft 114, whereas the other sheave 112 can slide freely along the shaft. The former is often called the fixed sheave 110 and the latter is called the mobile sheave 112. As the mobile sheave slides along the shaft, the distance between the conical faces changes and influences the size of the operating diameter around which the drive belt winds into the pulley thereby achieving the change in ratio of the CVT.
The torque supplied to the primary pulley by the engine is transmitted to the drive belt 106, then to the secondary pulley 104 by means of the friction between the conical surfaces of the sheaves and the sides of the drive belt 106. In order to prevent the drive belt from slipping onto the sheaves, a sufficient friction force must be sustained. This friction force is proportional to the axial force that is applied against the mobile sheave, forcing the mobile sheave to push the drive belt 106 towards the fixed sheave in order to pinch the drive belt between the two sheaves (110 and 112). The axial force, also called clamping force, is either provided by the result of force equilibrium within a tuned mechanical system, or is controlled using various motorisation or actuation systems.
The mechanical systems on which most CVTs rely to generate the clamping force are often comprised of springs, cams and centrifugal weights. Typically, the primary pulley takes advantage of the combination of a spring potential energy pushing the sheaves apart, and a group of weights 113 evenly distributed around the pulley shaft which exert a clamping force due to the centrifugal force generated by the rotational speed. The mobile sheave of the primary pulley is thereby pushed towards the fixed sheave, forcing the drive belt to wind up on a greater diameter which increases the tension in the drive belt. This additional induced tension forces the drive belt to wind down on a smaller operating diameter in the secondary pulley, therefore requiring the mobile sheave of the secondary pulley to yield and spread apart from the fixed sheave. As the mobile sheave 112 moves away under the additional tension induced in the drive belt 106, the clamping system (116 and 118) of the secondary pulley 104 opposes a reaction force produced by the mobile sheave displacement and the torque applied onto it. This axial force is often generated by a spring 118 compressing as the mobile sheave moves apart and a biasing mechanism redirecting the rotating torque axially towards the mobile sheave. A typical biasing mechanism comprises a cam and a sliding assembly 116. The torque received by the mobile sheave of the secondary pulley is transferred to the cam system. The cam system redirects the force axially and thereby increases the clamping force of the secondary pulley.
The performance of the CVT is measured by the rate at which its components will reach a state of force equilibrium when exposed to a load transition. However, these components (springs 118, cam 116 and weights 113) are subject to a hysteresis effect which considerably slows down the CVT's reaction to changes in vehicle speed. This phenomenon compromises the reliability of the CVT when quick shifting and back shifting reactions are required. Another common problem of the mechanical CVT is that is requires adjustments that are complex. Tuning the springs 118, masses 113 and choosing the right cam 116 in order to meet desired performance requires experience and time. Also, a setup tuning is only efficient for a limited range of performances. Therefore, the mechanical CVT that is precisely tuned to provide optimal acceleration will present limited performances in other applications such as handling sudden increase in torque.
Some CVT systems require using motorisation or actuation techniques instead of the conventional spring, mass and cam assembly. U.S. Pat. No. 8,682,549 to Roberge et al. describes an electronically controlled drive pulley 200 of a CVT, as presented in prior art FIGS. 2A and 2B. The electronically controlled drive pulley 200 has an electric actuation motor 202 adapted to rotate a plurality of operatively interconnected gears housed in a gearbox 204 to ultimately rotate a main actuation gear 206 at a desired speed.
As it is better seen in prior art FIG. 2B, the electric actuation motor 202 is operatively connected to the gearbox 204 that, itself, is operatively connected to the main actuation gear 206 via an elongated gear 208. The elongated gear 208 is provided with rather long teeth to accommodate a complete teeth-engaging axial displacement 210 thereon of the main actuation gear 206 that longitudinally moves along with an axially moveable sheave 212. The main actuation gear 206 is secured on a female threaded body 214. The female threaded body 214 upon rotation, transforms the rotation of the main actuation gear 206 into an axial movement that impacts the axial distance between the sheaves 212 and 216. It is the axial position of the axially moveable sheave 212 (distal in respect with the engine 218) that changes while the fixed sheave 216 remains axially at the same position. Any rotation of the electric actuation motor 202 is therefore transformed into a change in distance between both sheaves 212 and 216 of the drive pulley 200 to alter the transmission ratio of the CVT 10.
While such actuated systems provide precise and immediate control of the CVT ratio, a greater quantity of parts is however required. This results in large and complex assemblies, thus increasing the risk of a failure to occur.
Other CTV systems have a clutching system which allows disconnecting the pulleys from either the engine or the propulsion assembly. Centrifugal clutches and axial pressure clutches are the most common systems used. While this feature is essential to ensure a proper engagement of the engine power to the propulsion assembly as it is helpful to prevent too much torque from passing through the transmission, it often increases considerably the amount of inertia in the system. The power required to accelerate the transmission is increased and therefore the performance is diminished.
Therefore, there is a need for a system that easily allows precise and immediate control of the CVT ratio in order to provide performance over its full operative range without being complex to manufacture or maintain and without generating unnecessary inertia in the transmission.